The two major types of solar charge controllers are:

  1. Pulse Width Modulation (PWM) controllers
  2. Maximum Power Point Tracking (MPPT) controllers

As shown in the chart below, PWM controllers tend to be smaller and they operate at battery voltage, whereas MPPT controllers use newer technology to operate at the maximum power voltage. This maximizes the amount of power being produced which becomes more significant in colder conditions when the array voltage gets increasingly higher than the battery voltage. MPPT controllers can also operate with much higher voltages and lower array currents which can mean fewer strings in parallel and smaller wire sizes since there is less voltage drop.

PWM controllers need to be used with arrays that are matched with the battery voltage which limits what modules can be used. There are many 60 cell modules with maximum power voltage (Vmp) equal to about 30V, which can be used with MPPT controllers but are simply not suitable with PWM controllers.

To answer the question: Which is better, PWM or MPPT? All things being equal, MPPT is a newer technology that harvests more energy. However, the advantages of MPPT over PWM controllers come at a cost, so sometimes a less expensive PWM controller can be the right choice, especially with smaller systems and in warm climates where the MPPT boost is not as significant.

PWM vs. MPPT Solar Charge Controller Comparison

PWM Controllers MPPT Controllers
Array voltage is “pulled down” to battery voltage Convert excess input voltage into amperage
Generally operate below Vmp Operate at Vmp
Suitable for small module configurations Suitable for large module configurations that have a lower cost per watt
Often chosen for very hot climates which will not yield as much MPPT boost Provide more boost than PWM, especially during cold days and/or when the battery voltage is low

Every Morningstar PWM and MPPT solar charge controller is listed on the Morningstar Product Series page. Each listed product is hypertext linked to its product page that includes datasheets, operation manuals, and other helpful information.

PWM Charging

Traditional solar regulators featuring PWM (Pulse Width Modulation) charging operate by making a connection directly from the solar array to the battery bank. During bulk charging when there is a continuous connection from the array to the battery bank, the array output voltage is ‘pulled down’ to the battery voltage. The battery voltage adjusts slightly up depending on the amount of current provided by the array and the size and characteristics of the battery.

MPPT Charging

Morningstar MPPT controllers feature TrakStar technology, designed to quickly and accurately determine the Vmp (maximum power voltage) of the solar array. TrakStar MPPT controllers ‘sweep’ the solar input to determine the voltage at which the array is producing the maximum amount of power. The controller harvests power from the array at this Vmp voltage and converts it down to battery voltage, boosting charging current in the process.

Why Choose PWM Over MPPT

The preceding discussion of PWM vs. MPPT may cause some to wonder why a PWM controller would ever be chosen in favor of an MPPT controller. There are indeed instances where a PWM controller can be a better choice than MPPT and there are factors which will reduce or negate the advantages the MPPT may provide. The most obvious consideration is cost. MPPT controllers tend to cost more than their PWM counterparts. When deciding on a controller, the extra cost of MPPT should be analyzed with respect to the following factors:

1. Low power (specifically low current) charging applications may have equal or better energy harvest with a PWM controller. PWM controllers will operate at a relatively constant harvesting efficiency regardless of the size of the system (all things being equal, efficiency will be the same whether using a 30W array or a 300W array). MPPT regulators commonly have noticeably reduced harvesting efficiencies (relative to their peak efficiency) when used in low power applications. Efficiency curves for every Morningstar MPPT controller are printed in their corresponding manuals and should be reviewed when making a regulator decision. (Manuals are available for download on the Morningstar website).

2. The greatest benefit of an MPPT regulator will be observed in colder climates (Vmp is higher). Conversely, in hotter climates Vmp is reduced. A decrease in Vmp will reduce MPPT harvest relative to PWM. Average ambient temperature at the installation site may be high enough to negate any charging advantages the MPPT has over the PWM. It would not be economical to use MPPT in such a situation. Average temperature at the site should be a factor considered when making a regulator choice

3. Systems in which array power output is significantly larger than the power draw of the system loads would indicate that the batteries will spend most of their time at full or near full charge. Such a system may not benefit from the increased harvesting capability of an MPPT regulator. When the system batteries are full, excess solar energy goes unused. The harvesting advantage of MPPT may be unnecessary in this situation especially if autonomy is not a factor.

Why Choose MPPT Over PWM

Increased Energy Harvest:

MPPT controllers operate array voltages above battery voltage and increase the energy harvest from solar arrays by 5 to 30% compared to PWM controllers, depending on climate conditions.

Array operating voltage and amperage is adjusted throughout the day by the MPPT controller so that the array’s power output (amperage X voltage) is maximized.

Less Module Restrictions:

Since MPPT controllers operate arrays at voltages greater than battery voltage, they can be used with a wider variety of solar modules and array configurations. Moreover, they can support systems with smaller wire sizes.

Support for oversized Arrays

Unlike PWM controllers, MPPT controllers can support oversized arrays that would otherwise exceed the maximum operating power limits of the charge controller. The controller does this by limiting the array current intake during periods of the day when high solar energy is being supplied (usually during the middle of the day).

While energy from the array is capped or shaved off during the middle of the day, the oversized array is able to provide more power during teh early and late part of the day compared to smaller non-oversized array.